[v3]: Add Store method for getting the size of an item.

[TomAugspurger]

Zarr version

v3

Numcodecs version

n/a

Python Version

n/a

Operating System

n/a

Installation

n/a

Description

In #2400, we need a method for getting the size of some object from a Store to support .info. The cleanest way to do this is a new method in the Store API.

We could implement an inefficient version that basically does

def size(self, key: str) -> int:
    return len(await self.get(key))

Many backends will have more efficient methods for getting the size of a file / object without actually reading it.

Steps to reproduce

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Additional output

No response